Several critical Apollo mission maneuvers - for example, the rocket burn that launched the astronauts homeward from the moon - took place in lunar orbit over the moon's Farside hemisphere, out of sight and radio contact with Earth. Space historian David S. F, Portree describes a 1963 plan to ensure uninterrupted communication with Apollo spacecraft as they flew over the Farside.

Apollo-Saturn S-IVB stage. Image: NASA.The S-IVB rocket stage played several important roles in NASA’s 1960s and 1970s manned space programs. The 58.4-foot-long, 21.7-foot-wide stage, which comprised a single restartable J-2 rocket engine, a forward liquid hydrogen tank, and an aft liquid oxygen tank, served as the second stage of the two-stage Apollo Saturn IB rocket and the third stage of the three-stage Apollo Saturn V.Cutaway of S-IVB stage configured for use as Saturn V third stage. Image: NASA.The Saturn IB S-IVB’s J-2 engine would ignite at an altitude of about 42 miles and burn until it placed a roughly 23-ton payload into low-Earth orbit. After that, it would shut down and the spent stage would separate. The Saturn V S-IVB’s J-2, on the other hand, would ignite twice to accelerate the stage and its payload: once ...